It is often necessary in semiconductor processing to fill high aspect ratio gaps with insulating material. This is the case for shallow trench isolation, inter-metal dielectric layers, pre-metal dielectric layers, passivation layers, etc. As device geometries shrink and thermal budgets are reduced, void-free filling of narrow width, high aspect ratio spaces (e.g., AR>3:1, and particularly AR>6:1) becomes increasingly difficult due to limitations of existing deposition processes.
High density plasma (HDP) chemical vapor deposition (CVD), a directional (bottom-up) CVD process, is the method currently used for high aspect ratio gapfill. HDP CVD deposits more material at the bottom of a high aspect ratio structure than on its sidewalls. It accomplishes this by directing charged dielectric precursor species downward, to the bottom of the gap while simultaneously removing deposited material from the trench top through sputtering by the use of biased RF power applied to the substrate. Thus, HDP CVD is not an entirely diffusion-based or isotropic process.
However, there is still formation of cusps, also known as overhangs, at the entry region of the gap to be filled. These formations result from sputtering and redeposition processes. The directional aspect of the deposition process produces some high momentum charged species that sputter away material from within the gap. The sputtered material tends to redeposit on the sidewalls of high AR structures. As a result, the entry region of a high aspect ratio structure may close before bottom-up fill has been completed, leaving voids or weak spots within the structure. This phenomenon, known as “pinch-off,” is exacerbated in narrow features. The overhangs cannot be totally eliminated because non-directional reactions of neutral species and sputtering and redeposition reactions are inherent to the physics and chemistry of the HDP CVD processes.
Furthermore, as aspect ratios increase, the shape of the gap itself can contribute to the problem. High aspect ratio gaps often exhibit reentrant features, which make gap filling even more difficult. The most problematic reentrant feature is a narrowing at the top of the gap, wherein the side-walls slope inward near the top of the gap. For a given aspect ratio feature, this increases the ratio of gap volume to be filled to gap entry area seen by the precursor species during deposition. Hence voids or weak spots become even more likely.
In addition to undesirable formations inside the feature, a peak of dielectric material often called a “top hat” forms on the top surface of the substrate on either side of the features. Top hats are deposits of material in the shape of a peak that slopes downwards towards the entry to the gap. If not removed, the top hats may “shadow” subsequent deposition because they can restrict the angles of incidence with which the deposition species must approach the gap in order to achieve bottom-up fill. The shadowing effectively makes the aspect ratio of the gap even higher.
In some gap fill applications, particularly in the case of small features with high aspect ratios, a multi-step deposition/etch back process has been used in order to remove the overhangs, reduce the top hats, and facilitate void-free gap fill. For example, a deposition and etch process utilizing HDP CVD deposition and an aqueous HF dip for the etch back step has been used. However, this requires that the wafers be cycled between the plasma deposition system and the wet etch back system for up to five cycles. This results in a very inefficient and low throughput (e.g., about 3 wafers per hour (wph)) process for the gap fill.
In-situ multi-step deposition/etch processes have also been used to keep the entry to the gap from closing before it is filled. Such in-situ HDP CVD deposition and etch back processes are described, for example, in U.S. Pat. Nos. 6,335,261, 6,030,881, 6,395,150, and 6,867,086, the disclosures of which are incorporated herein by reference for all purposes. Some of these in-situ etch back processes use high-energy ions (e.g. helium from the HDP plasma) to create a significantly anisotropic sputter etch. Other in-situ etch back processes use chemically-reactive etch gases (e.g. nitrogen trifluoride, NF3) to create a significantly isotropic dry plasma etch.
In many instances, these sputter etch and reactive plasma etch processes damage the underlying structure of the features on the substrate. For example, in STI structures, a SiN liner is often deposited to enhance electrical isolation between various device elements and improve device performance. The liner can be eroded via several chemical or physical pathways, resulting in compromised device performance (usually demonstrated by higher leakage current).
While these in-situ multi-step deposition and etch back processes have improved high aspect ratio gap fill capabilities, dielectric deposition processes that can reliably fill high aspect ratio features of narrow width, particularly very small features (e.g., about 0.1 um gap width) with aspect ratios of about 6:1 or more, without leaving voids, continue to be sought. A method to reliably fill the features without causing detrimental damage to the underlying structure is still needed.